fix(drivers): reject whitespace in mount fields

[elezar]

Summary

Make shared driver-mount validation non-transforming. Mount fields with surrounding whitespace are now rejected, while target normalization is an explicit operation used only where semantic comparison requires it.

Related Issue

Follow-up to #2034, which triggered this validation investigation.

Changes

• Change shared mount validators to return validation success without rewriting values
• Reject surrounding whitespace in mount sources, subpaths, and container targets
• Add explicit normalize_mount_target usage for duplicate-target detection
• Preserve validated Docker and Podman mount values unchanged
• Remove implicit source trimming from volume availability checks

Testing

• mise run pre-commit passes (all formatting, linting, Clippy, and workspace checks passed; the Rust test task hit an unrelated local macOS linker error: library not found for -lSystem)
• mise exec -- cargo test -p openshell-core driver_mounts (7 passed)
• mise exec -- cargo test -p openshell-driver-docker -p openshell-driver-podman driver_config (14 passed)
• Focused Clippy passes with warnings denied
• E2E tests added/updated (not applicable)

Checklist

• Follows Conventional Commits
• Commits are signed off (DCO)